Lol, get ready for everyone to roll an Assassination Rogue for PvP, as if they weren't already on their way to being Flavour of the Season. 30 yard auto-attacks and poison procs are pretty much the main factors in Assassination DPS and now they can do it from range. Can't wait.

Not really. The overall changes are a buff for the intended but seldom used arcane playstyle (stack reseting) and a serious nerf for the currently most common, unintended and strongest playstyle (scorch-weaving). It will probably make scorch-weaving impossible.

GC recently tweeted they were going to make arcane blast stack less and be worth more each stack so that ramp up wasn't so punishing, especially because of the scorch fix. Don't take it too far out of context, a max stack AB should be same as it was before but arcane shouldn't feel as nerfed in 5.2 as before losing ability to keep AB stacks with scorch weaving. mage damage for arcane will probably still be slightly down in 5.2 vs 5.1 even with this change since arcane mages will stlll be clearing more often.

Magnificence of Leather (New) Create a Magnificent Hide from Exotic Leather once a day. In the process, your knowledge of Pandaren Leatherworking may increase, resulting in the discovery of a new pattern. Shares cooldown with the Magnificence of Scales.
Magnificence of Scales (New) Create a Magnificent Hide from Prismatic Scales once a day. In the process, your knowledge of Pandaren Leatherworking may increase, resulting in the discovery of a new pattern. Shares cooldown with the Magnificence of Leather.

Does this mean we will only be able to create 1 Magnificent Hide per day or will this be in addition to making Magnificent Hide from 50 exotic leathers?

A lot of the raid ability changes look like tweaks to encounters that they've already previewed, not additions. I wonder if we're getting another round of 10-man testing of those 5 bosses, or if they're going to put up some of the other encounters soon?